In many applications it is necessary to fix a foil or a film in a tight and detachable manner to a base of plastic material which mainly consists of oriented material. In the following, without any restrictive significance, the designation film is used for the foil or film which is to be fixed to the base. The structure of the film varies from occasion to occasion and is constituted in certain applications of a single-layer film, e.g. of plastic material or of metal, and in other applications of a laminated film. In laminated film the layers consist, for example, simply of plastic material when the composition of the material as a rule varies from layer to layer whilst in other applications one or more of the layers consist of metallic material, of barrier material etc. The side of the film which is to be fixed to the base, however, is always made up of a material suitable for being fixed to the base by means of heating.
It has been found, surprisingly, that it is not possible in the case of certain plastic material, e.g. polyethylene terephtalate (PET) or material similar to it, to fix film to a base when this consists of oriented plastic material and to form a bond therewith which is both tight and detachable. It is found that with previously known and conventionally applied fixing techniques, where the film is pressed to the base by mechanical means and at the same time energy is supplied to the film, it is certainly possible to fix the film to the base in a tight bond, but that the tight bond sought is obtained only if the energy supply (heat supply) takes place on a scale which causes the film to be fixed to the base so strongly that subsequently it cannot be torn off. This means, of course, that the combination of film of the aforementioned type and oriented material in the opening part of a container of plastic material cannot be used for a package (container) where easy opening is required, i.e. containers for foodstuufs, such as dairy products. A further problem is that the film becomes defored when the energy supply is so great that a tight bond is achieved.
The product obtained, for example, a cup with oriented material at the opening is closed by means of a film, is therefore not acceptable, since on the one hand the film is defored to an extent which makes the container unacceptable to the consumer, whilst on the other hand the film sticks to the edge of the opening so strongly that it cannot be torn off.